In U.S. patent application Ser. No. 051,853 filed on May 19, 1987, a catalyst system is provided for producing stereo regular polymers. More particularly, it was found that polymers having an isotactic index of at least 96 percent could be prepared in high yield at high production rates by polymerizing an alpha-olefin in a low pressure gas phase fluidized bed process at temperatures in excess of 50.degree. C. employing a catalyst system comprising (i) a catalyst precursor, which includes magnesium, titanium, halogen, and an inside electron donor, i.e., a polycarboxylic acid ester containing two coplanar ester groups attached to adjacent carbon atoms; (ii) a hydrocarbylaluminum cocatalyst; and (iii) an outside electron donor or selectivity control agent, i.e., a silicon compound containing a silicon-oxygen-carbon linkage wherein the atomic ratio of aluminum to silicon is in the range of about 0.5:1 to about 100:1 and the atomic ratio of aluminum to titanium is in the range of about 5:1 to about 300:1. An alternative catalyst system uses an outside electron donor similar to the inside electron donor, i.e., the polycarboxylic acid ester described above, to the same end.
Although the gas phase activity of these catalyst systems is more than satisfactory, certain end uses for propylene based resins require much lower resin ash values than are obtained with this catalyst system. Thus, a more active catalyst system would be desirable, particularly in the production of polymer fibers, film, and medical packaging.